Unlimited Ammo
Go to Multiplayer and host a newgame. It can be any connection.
When you get to change the game settings, one of the menus
let's you set unlimited ammo. Start the multiplayer game and
then quit it. Load an old game or start a new one and you
should have unlimited ammo, as well as unlimited grenades.
You only have to do this once each time you start the game.